40: Champernowne's constant

40_champ_constant/main.go

@@ -0,0 +1,36 @@
+package main
+
+import (
+	"fmt"
+	"math"
+	"strconv"
+)
+
+func main() {
+	p := 1
+	for i := 0; i < 7; i++ {
+		n := int(math.Pow(10, float64(i)))
+		fmt.Println(n)
+		p *= getDigit(n)
+	}
+	fmt.Println("num=", p)
+}
+
+func getDigit(n int) int {
+	var r, s, k int
+	for s < n {
+		r = s
+		k++
+		s += k * 9 * int(math.Pow(10, float64(k-1)))
+	}
+	h := n - r - 1
+	t := int(math.Pow(10, float64(k-1))) + h/k
+	p := h % k
+	for i, s := range strconv.Itoa(t) {
+		if i == p {
+			ret, _ := strconv.Atoi(string(s))
+			return ret
+		}
+	}
+	return 0
+}

40_champ_constant/main_test.go

@@ -0,0 +1,37 @@
+package main
+
+import "testing"
+
+func Test_getDigit(t *testing.T) {
+	type args struct {
+		n int
+	}
+	tests := []struct {
+		name string
+		args args
+		want int
+	}{
+		{
+			"12",
+			args{12},
+			1,
+		},
+		{
+			"13",
+			args{13},
+			1,
+		},
+		{
+			"15",
+			args{15},
+			2,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := getDigit(tt.args.n); got != tt.want {
+				t.Errorf("getDigit() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}